If you can't get the GBoard / Google Voice Services to work then I can suggest two other methods.
First is to use Sayboard. For my use, it's fine. Not as accurate or fully featured as GBoard / Google Voice but good enough for the occasional voice transcription.
Second, you can also install Google Recorder (no need to also install any Google Play packages), revoke network access, record / transcribe in that app and then share or copy - paste the results of the transcription into your note taking, text messaging, etc. app of your choice. Ads an extra step but just as accurate as the GBoard / Google Voice setup.